This application claims priority to U.S. patent application Ser. No. 61/502,397 , which was filed Jun. 29, 2011 , The priority application is hereby incorporated by reference in its entirety into the present application.
Magnetic bearings support a shaft by magnetic levitation, without physical contact, thereby eliminating mechanical wear and exhibiting very low friction. One disadvantage to magnetic bearings, however, is their inability to accommodate high dynamic loads due to their limited load capacity. During peak transient load events, the bearings are unable to control rotor motion causing the rotor to contact the bearings, resulting in significant damage thereto. To account for this, magnetic bearing systems often employ one or more coast-down bearings, also known as auxiliary, backup, secondary, or catcher bearings or bushings. The coast-down bearings are designed to support the shaft in the event of a failure or shutdown of the magnetic bearing system, while the shaft is slowing down (i.e., coasting down). This prevents the shaft from impacting and damaging the magnetic bearings.
Conventional coast-down bearings provide a clearance between the bearing and the shaft. During normal operation, the magnetic bearings support the shaft and hold it within this clearance such that the shaft rarely, if ever, contacts the coast-down bearing; thus, the coast-down bearing is typically stationary with respect to the shaft during such normal operation. When the magnetic bearing system shuts down or otherwise fails to support the shaft in the clearance, the shaft is constrained by the coast-down bearing, preventing damage to the magnetic bearing. When the shaft impacts the coast-down bearing, however, the rotational motion of the shaft is not always stable and very high stresses and dynamic loads are often endured by the coast-down bearing, which limits its useful life.
One way in which dynamic loading of the coast-down bearing can be experienced is by the shaft whirling during coast-down, that is, when the shaft “orbits” around the interior of the coast-down bearing. Whirling generally has two modes, forward whirl, where the shaft orbits in the same direction as it rotates, and backward whirl, where the shaft orbits in the opposite direction from which it rotates. Backward whirl can be particularly problematic for coast-down bearings.
What is needed then is a coast-down bearing that minimizes dynamic loading caused by whirling, thereby increasing the useful life of the coast-down bearings.